My best (stupid) guess is weapons. US gov has a ton of weapons. Already sells tons of weapons. Now the prices will always remain stable. Other countries would love to know that 30 million usd would always be able to buy an f15. Other countries declaring war will increase the value of the USD, as buying weapons from us government will decrease amount of money in circulation.
But doing something just for the sake of finances…it doesn’t add anything to the world. The original question was “what do you pick?” and I don’t like the idea of picking an energy-intensive activity that doesn’t help the world in any way.